1. Field of the Invention
The invention relates to electromagnetic interference reduction. More specifically, the invention relates to a system and method for reducing the internal and external effects of electromagnetic interference generated by network devices and devices operating in the same or similar frequency ranges.
2. Description of Related Art
Telecommunications and data networks employ devices which act as nodes within the network that route, switch, or otherwise manage the various signals being sent along multiple optical fibers and other types of lines. As the need for faster and greater data transfer increases, so does the complexity of these networks as well as the network devices therein.
These network devices, like most electronic devices, are subject to the regulations of the Federal Communications Commission (FCC) and other organizations worldwide because they generate radio frequency (RF) signals in the form of electromagnetic interference (EMI). In many other conventional electronic devices (including prior conventional telecommunications devices), shielding was provided to retain the RF interference within the cabinet or housing of the device. In such an arrangement, one or more of the components of the device generates a stray RF signal as interference, and the interference strikes and bounces off the shielding and is trapped inside the housing. This shielding can be in the form of metal gaskets that line the perimeter of the housing, or can take the form of a sealed metal box-like structure known as a Faraday cage. In some network devices, the electronic components of the device are arranged on cards called line modules which are stacked or hung either vertically or horizontally in a chassis or cabinet. The backs of the line modules may be provided with a plate of metal which can act as a shield against RF interference. Since a given line module has its own metal plate and it faces the back of an adjacent line module, stray RF signals have heretofore been relatively contained.
The conventional shielding approach, however, is not practical for use with modern network devices which process signals at extremely high frequencies and in high density (such as, for example, the Core Director™ switch made by CIENA Corporation), for several reasons. First, it is impractical to seal the cabinet or housing of such a device, because routine, hot-swappable maintenance of the components requires frequent entry into the housing. Providing a seal that would reseal perfectly after every opening of the housing would be difficult and impractical.
Second, even if a seal were provided for modern network devices, it would need to be extremely tight. The clock speed of some of the components of a such networking devices are in the range of 1–40 GHz (e.g., serializing/deserializing chips or SERDEs). Because the speed of the components is so high, the frequency of the resulting RF interference is equally high, and the wavelength of the interference is extremely small. With such a small wavelength of interference, the shielding would need to be extremely tight, otherwise the RF interference would escape from the smallest of gaps or holes in the shielding.
Finally, even assuming such a tight, resealable shielding were practical and effective to prevent the device from emitting 1–10 GHz RF interference, the shielding would simply reflect the interference back into the housing of the network device as it would in a conventional RF shielding application. In this case, however, the reflected EMI noise would impede the functioning of the device (or, at very least, slow the device down to an unacceptable level). That is, the use of effective shielding would cause the network device to interfere with itself.
Thus, as network devices have become faster and more complex, traditional RF shielding techniques have become less and less relevant and effective. In the case of state of the art optical network devices such as the Core Director™ switch, RF shielding is not a complete solution. Another means or method of dealing with the RF noise generated by these extremely high speed devices—particularly in the 1–10 GHz range—needed to be developed. In addition, network devices must conform to the Network Equipment Building System (NEBS) standards limiting the overall size or “footprint” of the device. Thus, any modifications to a network device could not increase its footprint.